THE TEETH OF PERISSODACTYLE UNGULATA. 321 



From the grinding surface of the crown there dips in a 

 deep fold of enamel, forming a cul de sac. As this pit does 

 not extend the whole depth of the crown, and the in- 

 cisors of a horse are submitted to severe wear, the fold 

 eventually gets worn away entirely, and the worn surface 

 of the dentine presents 110 great peculiarity. But as this 

 wearing down of the crown takes places at something like a 

 regular rate, horse dealers are enabled to judge of a horse's 

 age by the appearance of the mark upon the different incisors. 

 The " mark " exists in Hipparion, but not in the earlier 

 progenitors of the horse. 



FIG. 136 0). 



FIG. 137 



A. horse attains to its adult dentition very slowly ; the 

 first permanent incisors appear about the end of the third 

 year, and the other two pairs follow at intervals of about 



Incisors of the Horse, showing the marks at various stages of wear. 



